A computer-to-plate (CTP) device 204, shown in FIG. 2, is used for direct imaging on printing plates. The plates are loaded in a magazine or cassette and delivered one by one to be exposed by the imaging device. Alternatively the plates can be provided by an automatic plate loader (APL) 104, wherein a plates stack 108 is inserted into the APL as is shown in FIG. 1. The plates 304 provided in the cassette or in a plate stack 104 are usually separated by interleaf paper 308, shown in FIG. 3, interposed between the plates to prevent the emulsion-covered surfaces of the plates from being damaged.
In the course of imaging plates, the plate placed at the top of the stack is picked and transferred to the exposure area for imaging. When an interleaf paper (slip-sheet) is at the top of the stack, the paper is picked and disposed of, before picking the plate. There is a need for discriminating between plate and the interleaf paper are used, to correctly identify the topmost object on the stack.
U.S. Pat. No. 6,825,484 (Burkatovsky) describes a discriminating device based on measurements of the light reflections from the surfaces with different roughness. For example, discrimination between paper and non-covered by emulsion printing plate will be reliable due to substantially different roughness of paper as opposed to a smooth and glossy plate metal surface. But discrimination between paper and emulsion covered plate will often be inaccurate due to the small difference between their roughness properties.
Another method for discriminating the slip sheets and emulsion covered printing plate described in U.S. Pat. No. 7,157,725 (Kawamura). This method is based on the difference between absorbance (reflectance) of a slip sheet and a plate, irradiated by light of 570-740 nm wavelengths.
Reflectance of emulsions and papers produced by different manufacturers may vary substantially. An example of reflections from papers and emulsions of different manufacturers is shown in FIG. 6. X-axis depicts lighting source current or irradiating intensity T and Y-axis depicts the reflection intensity represented by photosensor output voltage Vr. The reflections from paper slip sheet and emulsion covered plate manufactured by the first manufacturer and measured at predetermined height and light source current is shown by lines 604 and 608 respectively. Reflections from paper slip sheet and emulsion manufactured by the second manufacturer and measured at the same height and light source current is shown by lines 612 and 616. The threshold Vth1 for discrimination between paper and emulsion produced by the first manufacturer according to Kawamura et al., should have value greater than emulsion reflection and less than paper reflection values. While irradiation caused by the same light source current the reflections from paper and emulsion produced by the second manufacturer are smaller. This might happen due to different processes applied for emulsion covered printing plates and slip sheet by different manufacturers. In this case the chosen threshold Vth1 will be greater than paper and emulsion reflections and media discriminating will be impossible.
It should be noted that not only manufacturer media variations and differences between the batches of media lead to reflection deviations. Changing parameters such as distance to media, light source, ambient light are also impact on reflections thus making difficult to practical implementation of the method suggested by Kawamura et al.
The purpose of this invention is to improve the paper slip sheet and emulsion covered plate discrimination capability.